Segment Reporting - Identifiable assets by industry segment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Assets	$ 282,704	$ 246,872
Operating Segments [Member]
Segment Reporting Information [Line Items]
Assets	282,704	246,872	$ 243,215
Operating Segments [Member] | Marketing [Member]
Segment Reporting Information [Line Items]
Assets	134,745	107,257	96,723
Operating Segments [Member] | Transportation [Member]
Segment Reporting Information [Line Items]
Assets	29,069	32,120	35,010
Operating Segments [Member] | Oil and Gas [Member]
Segment Reporting Information [Line Items]
Assets	425	7,279	8,930
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Assets	$ 118,465	$ 100,216	$ 102,552